In a universal mobile telecommunications system (UMTS) network, high speed downlink packet access (HSDPA) or high speed uplink packet access (HSUPA) is employed in order to improve spectrum efficiency and a data rate so that characteristics of a wideband code division multiple access (W-CDMA) based system can be maximized. In such an UMTS network technology, long term evolution (LTE) has been discussed to obtain a higher data rate, a lower delay, and the like.
In a 3GPP system, a fixed band of 5 MHz is used in general to implement a transmission rate of 2 Mbps at maximum in downlink. Meanwhile, in an LTE system, transmission rates of 300 Mbps in downlink and 75 Mbps in uplink at maximum can be implemented using variable bandwidth between 1.4 MHz and 20 MHz. In addition, in a UMTS network, in order to obtain a wider bandwidth and a faster transmission rate, a post-LTE system is also discussed (e.g., LTE-Advanced (LTE-A)). For example, in LTE-A, it is anticipated that the maximum system bandwidth of 20 MHz in the LTE specification is expanded to, approximately, 100 MHz. In addition, it is anticipated that the maximum number of transmit antennas set to 4 in the LTE specification is expanded to 8.
In an LTE system, data are transmitted/received using a plurality of antennas, and a multiple input multiple output (MIMO) system has been proposed as a radio communication technique for improving a data rate (spectrum efficiency) (see e.g., Non-patent Literature 1). In the MIMO system, a plurality of transmit/receive antennas are provided in a transceiver, and different transmission information sequences are simultaneously transmitted from different transmit antennas. Meanwhile, at the receiver side, the information sequences transmitted simultaneously are separately detected in consideration of a fact that different fading variations are generated between transmit/receive antennas, so that it is possible to increase a data rate (spectrum efficiency).
In an LTE system, there are defined single-user MIMO (SU-MIMO) in which overall transmission information sequences simultaneously transmitted from different transmit antennas belong to the same user and multiple-user MIMO (MU-MIMO) in which transmission information sequences simultaneously transmitted from different transmit antennas belong to different users. In both the SU-MIMO transmission and the MU-MIMO transmission, an optimal precoding matrix indicator (PMI) is selected from a codebook in which a plurality of phase/amplitude control amounts (precoding matrix (precoding weight)) to be set for the antennas of the transmitter at the receiver side and a plurality of PMIs corresponding to the precoding matrix are defined for each rank, and the optimal PMI is fed back to the transmitter. In addition, a rank indicator (RI) indicating the optimal rank is selected and fed back to the transmitter. At the transmitter side, the precoding weights for each transmit antenna are specified based on the PMI and the RI fed back from the receiver, and the precoding is performed, so that the transmission information sequences are transmitted.